The water supply would cut off to the cold feed showert first, as the lower cold feed to the HWC will continue to feed and therefore continue to heat up. This means there would not be a hot and cold mix in the thermo valve.
You need a 50 gallon Cistern in the roof to feed a power shower and a HWC, especially if the shower is pumped.
Try and install the cold feed to the shower at the lowest point of the cistern, with the feed to the HWC 150mm above that.
You will also need a minimum head from the tank to shower outlet of 1m, or 150mm if it is a pumped shower.
Good luck
